Peter, no need for apologies. As Ron just said, "Notation of hidden sets has always been a bit awkward IMO". Since I've been bitten before on the notation, I was sensitive about being caught off balance again.

I agree that my solution sucks! _ _

Ron, how about: (it uses the conjugate on <1>)

(3=5)r1c5 - (5)r1c13 = (5-1)r3c1 = (1)r1c1 => r1c1<>3

This gets around reading Peter's chain from r-to-l and performing -(5)r1c3 instead of -(5)r1c13.